Pressure sensors that can operate at high temperatures have uses in numerous industrial sectors, and have been used in gas turbine engines, coal boilers, furnaces, and machinery for oil/gas exploration. A number of optical approaches have been reported in the past, utilizing Fabry-Perot and other interferometers. Typically, these use an optically reflective cavity on the end of a fiber optic cable; the cavity size changes with pressure, causing measurable interference changes in reflected light. A thin diaphragm is typically used as the reflective surface. Operating temperatures up to 800° C. have been achieved with sapphire membranes. An interferometer-based sensor has also been fabricated inside a fiber optic cable. Another sensing technology uses Bragg gratings, which are photo inscribed into fibers, and used to trace wavelength shifts caused by pressure and temperature changes at temperatures exceeding 350° C., and potentially over 1,500° C. Piezoresisitve pressure sensors with diaphragms made from silicon carbide, and more recently even Si, have been reported to operate at 600° C. Sapphire membranes have also been used in this context.
Microdischarge-based pressure sensors can complement this portfolio by offering an electrical transduction and structural simplicity. Microdischarges are miniature, localized plasmas (and may include, more generally, arcs or sparks) created in gas ambients between electrodes which, due to their size, demonstrate characteristics different from those of plasma regions created on a larger scale. Microdischarges have been explored for applications in a variety of micro total analysis systems, including microscale optical emission spectroscopy systems for chemical sensing. Devices utilizing microdischarges are well suited for high temperature operation as the electrons have average thermal energies exceeding 3 eV (34,815 K) away from the cathode and small populations of very high energy electrons with thermal energies exceeding 400 eV near the cathode. Ions have thermal energies exceeding 0.03 eV above ambient (644 K) in a 23° C. (296 K) ambient environment. These temperatures allow the species to be only minimally affected by a high or low temperature ambient, making it possible for microdischarge-based devices to operate at temperatures in excess of 1,000° C. and potentially down to cryogenic temperatures. The targeted performance range for this work is 200-1,000° C., but some baseline studies at room temperature are included. With regard to pressure sensors, microdischarge-based devices offer the possibility of structural simplicity and a direct electrical readout.
Microdischarge-based pressure sensors of the type disclosed herein operate by measuring the change in spatial current distribution of microdischarges with pressure. The targeted pressure range is 10-2,000 Torr, as might be encountered in a variety of manufacturing applications. Pressures outside of this range can also be measured. Additionally, pressures within this range have not been conventionally measured by technologies involving discharges-based devices. As gas pressure increases, the mean free path of ionized molecules is reduced and consequently, the breakdown and discharge characteristics are altered. Microdischarge-based pressure sensors are fundamentally different than ion gauges, which are not effective at atmospheric pressure because the small mean free path of the created ions, 20-65 nm, makes them difficult to detect at the collector.
A microdischarge-based pressure sensor using a planar electrode structure is known from the inventors' prior work published in A Harsh Environment, Multi-Plasma Microsystem With Pressure Sensor, Gas Purifier, and Chemical Detector, Proc. IEEE Int. Conf Micro Electro Mech. Syst., Kobe, Japan, 2007, pp. 115-118. The pressure sensor disclosed in this article utilizes a single circular planar anode partially surrounded by concentric cathodes. All of the electrodes are formed as thin-film coplanar elements on a glass substrate. Short duration dc pulses are applied to create pulsed microdischarges between the anode and all of the cathodes, and the current distribution between two cathodes is measured and used to determine ambient pressure.